MS Outlook does not normally block PPS/PPT/PPSX/PPTX files. Try this (based on XP but you should be able to adapt to suit OS):
Start Menu > Settings > Control Panel > Folder Options.
File Type tab > scroll to and select PPTX extension.
Click "Advanced" button.
If you see a box entitled "Confirm Open After Download" UNCheck it and click Apply.
Close Folder Options dialog and try again.
There is another known issue that concerns compression of attachments, and may be applicable to you. How to disable "Max Compression" of attachments:
MS Outlook > Help menu > About Microsoft Outlook.
Click "Disabled items" button.
Select "max compression" line > click "Enable" button.
Tools menu > Options > "Max Compression" tab.
Choose "Do not compress attachments at all" > Click "OK".
Close and re-open Outlook and test again.
Let us know if that helps. We can suggest more options, such as trying it with "DDE" off in the Folder Options > File Types for the PowerPoint-related file types.